Devices for the automated release of liquid medicaments are normally used with patients who have a continuous and in the course of the day varying need of a liquid medicine which can be administered by infusion. Specific applications are, for example, certain pain therapies, cancer therapies and the treatment of diabetes mellitus, in which computer controlled infusion pump devices are used. Such devices are particularly useful for ambulatory therapy, and are generally carried attached on or near the body of a patient. The medicine reservoir often comprises medicine supply sufficient for one or several days. The liquid medicament is supplied to the patient's body from the medicine reservoir through an infusion cannula or an injection needle.
Ambulatory infusion pump devices are typically of the syringe driver type, where the liquid medicament to be administered to the patient is stored in a cylindrical glass cartridge or ampoule acting as the liquid medicament reservoir, and is conveyed to the body of the patient by displacing a plunger within the cylinder. An example of such an infusion pump device is disclosed for example in WO 01/83008 A1. A cylinder of the dosing unit comprises the complete reservoir of liquid medicament of the infusion pump device. A plunger plug arranged in the cylinder is unidirectionally displaced along the cylinder axis by a drive system via a shaft or threaded spindle.
EP 2163273 A1 of the applicants, the disclosure of which is hereby included by reference in its entirety, discloses a piston pump based dosing unit for an infusion pump device with a 4/3 or 3/3 way valve system arranged at a front end of the cylinder of the dosing unit. A plunger arranged in the cylinder of the dosing unit can be bidirectionally displaced along the cylinder axis by a spindle drive system. In one state of the valve, an inlet conduit fluidly connected to the primary reservoir is fluidly connected to the cylinder cavity, and an outlet conduit fluidly connected to the infusion tubing is disconnected from the dosing unit. This state of the valve is applied during the refill mode, when the dosing unit retracts the plunger and sucks liquid medicament from the primary reservoir into the pump cylinder/secondary reservoir. In a second state of the valve, the cylinder of the dosing unit is fluidly connected to the outlet conduit, thereby establishing a fluid connection to the body of the patient. The inlet conduit is disconnected from the dosing unit. This valve state is applied during the pump mode, when liquid medicament is conveyed from the secondary reservoir in the cylinder of the dosing unit to the subcutaneous tissue of the patient. The valve thus either allows the dosing unit to retrieve liquid from the primary reservoir, or to convey liquid from the secondary reservoir of the dosing unit toward the patient.
The valve is realized as a rotatable cylinder acting as a valve member, mounted in a fixed valve seat. The cylinder member of the valve is frictionally connected to the plunger. By rotating the plunger along the cylinder axis, the actuating means of the plunger indirectly actuate also the valve member, by rotating the cylinder/valve member within the stationary valve seat. Thus no separate actuator is needed for the valve. Furthermore the valve system and the piston pump are coupled such that in the pump mode, the valve will automatically be in the pump state, and in the refill mode, the valve will automatically be in the retrieving state. Thus also no additional control means is needed for the valve.
In order to rotate the cylinder and to switch the valve, the plunger has to exert a rotational torque on the cylinder member. At the same time the rotational torque exerted on the plunger by a drive unit has to be translated into a linear displacement of the plunger along the cylinder axis. For that purpose the plunger shaft is provided with an outer thread that interacts with an inner thread provided on the cylinder. The dosing unit is designed such that the static frictional force between plunger and cylinder member (including friction between plunger plug and cylinder wall and friction of the thread) is larger than the static friction between cylinder member and valve seat. When the drive unit rotates the shaft into one direction, the cylinder is frictionally coupled to the rotating plunger and rotates in the valve seat until finally reaching an end position, where its further rotation is mechanically blocked by a stopper. The plunger is now frictionally decoupled from the cylinder member, and any further rotation of the plunger is translated by the threads into a linear displacement along the cylinder axis. When the rotation of the plunger is reversed, the cylinder member is no longer rotationally blocked in the valve set, and plunger and cylinder member are frictionally coupled again. The cylinder member thus rotates in the valve seat in the reverse direction, actuated by the drive unit via the plunger, until a second stopper is reached, corresponding to the second valve state. The plunger is decoupled again from the blocked cylinder member, and is now linearly displaced along the cylinder axis in the opposite direction. The design of such a combined piston pump/valve system is named “valve before plunger” design, since the valve is actuated before the plunger is actuated.
In an infusion pump device as discussed above, all parts that come into contact with liquid medicament, as well as parts that are subject to friction, may be arranged in a disposable subunit. Prior to use of the infusion pump device, a fresh disposable subunit is coupled to a reusable subunit, comprising for example the electronics, the drive system, the battery, and all other parts that are not prone to contamination or wear, and can be used over a longer time period.
Since the spindle that actuates the plunger of the pump is typically made from a polymer material, and thus would be subject to wear if used over a long time period, it is advantageously also part of the disposable subunit. However, if a simple threaded rod coupled to the plunger would be used as the spindle of the spindle drive, the length of the disposable subunit would depend on the plunger position. Such a design would complicate the coupling of the disposable subunit to the reusable subunit, namely the coupling of the spindle to the drive motor.
In the dosing unit shown in EP 2163273 A1, the length of the disposable subunit is hold constant. The plunger spindle of the dosing unit comprises a plunger shaft attached to the plunger, and a coaxial plunger driving rod arranged in a longitudinal bore along its axis of the plunger shaft. The driving rod can be linearly shifted along the longitudinal bore. The cross-section of the driving rod and the longitudinal bore are chosen such that a rotational torque is effectively transmitted from the driving rod to the plunger shaft. The coupler element can be coupled to a driving unit of a reusable subunit. The linear displacement of the plunger during the spindle actuation is compensated by the two-part spindle. While the plunger shaft is linearly shifted together with the plunger, the driving rod remains on place in regard to the drive unit. The transmission of the rotational torque from the drive unit to the plunger shaft is effected by the rotational coupling between plunger shaft and driving rod.
A spindle drive, as it is also present in the above-referenced dosing unit, unavoidably has a certain thread lash. When spindle rotation is reversed, the flanks of the inner thread and the outer thread are slightly shifted in regard to each other along the longitudinal axis. In a standard single-reservoir syringe-type infusion pump, thread lash is not relevant, since the spindle drive in unidirectional. Thus after priming of the pump system there is no reversal of the spindle rotation direction. Thread lash can be potentially detrimental to metering precision for secondary-reservoir piston pump dosing units, where the rotation direction is repeatedly reversed after priming. After switching the rotational direction of the plunger, for a small rotation angle of the plunger the threads are not coupled, resulting in a rotation of the plunger without linear displacement. Furthermore the linear force exerted on the plunger may have to counteract an opposite force due to a pressure differential in the cylinder cavity. Depending on the circumstances this can lead to a small linear displacement of the plunger within the cylinder, without rotation. As a result, the metering precision of such a dosing unit may be restricted.
This precision reducing effect of the thread lash is particularly relevant when the rotation angle and/or the linear displacement of the plunger are used to determine the position of the plunger plug within the cavity and/or the volume of retrieved/conveyed liquid volume. Although such a metering measurement method is very precise, and can take into account various effects, it cannot counterbalance thread lash, since thread lash cannot be detected such a method.
Counter spindle drives are used to avoid thread lash in high precision linear motors, for example for machining devices such as lathes. However, such complex drives are not applicable for infusion pump devices, since they are too voluminous and complex.